It appears that the Let Us Calculate Your Expense Deductions for You prompt will only appear when you select that you have rented out the unit for the entire year. This is allowable even if the unit was not rented out the entire year if it's the first year you've rented the unit, as noted previously. If you do anything else, TurboTax will not provide the "Let Us Calculate" prompt.
Frankly, I don't trust TurboTax to do this calculation properly. I originally was going to allow TurboTax to do the calculation and had selected this option and entered the SqFt ratio for the rental portion on my home. When I noticed issues and tried to change the selection to "No, I'll do the math and enter the rental-only portion", I couldn't get the prompt back. That's because I'd changed my response to the "entire year" question to "no". Once I changed it back, I got the prompt again.
